Nightshroud, known as Darkness in the Japanese version, is a malicious spirit and the progenitor of the World of Darkness in Yu-Gi-Oh! GX. He has the ability to possess humans, and is partially responsible for the Abandoned Dorm incident. He briefly appears as a minor adversary in the first season and second season, and later returns as the main antagonist in the fourth season and final threat of the GX series. He shares many similarities with Yami Bakura, in that he initially requires a host, and is a recurring villain who eventually turns out to be the series' final antagonist.

Nightshroud's character design was overseen by Kenichi Hara. While possessing a host, Nightshroud wears a tight-fitting overcoat with raised shoulders and waistline and gold and green trimmings.

His dark dragon mask, resembling the head of "Red-Eyes Darkness Dragon", is completely black with the exception of its blue eye-pieces and a red stone in the center of the forehead. In this incarnation, the bangs closest to his face are positioned in front of the mask.

Immediately before Yusuke's fall, Nightshroud's energy form is glimpsed as an immaterial spirit, with a body swathed in black smoke. He has two long, gangly arms ending in claws, and no legs. His true form is a cloaked skeleton with glowing blue eyes resembling a Grim Reaper. He wears grey and red robes with gold trim, and has a dragon-shaped skull for a head with horns at the back of it, with the same shape as the mask his previous hosts wore. In this form he does not use a Duel Disk - instead, he grows five wings, each serving as one Monster Card Zone and one Spell and Trap Card Zone apiece. His cards materialize directly into his hands - his Deck is unseen.

Nightshroud is a calm individual with the belief that individuality, hope, and bonds are what bring about people's suffering. He believes himself to be a "savior" and that complete peace can be brought into people through complete nullification (giving people neither hope nor despair). Nightshroud rarely loses his composure in conversations or Duels unless he is tricked or caught off-guard, especially if it relates to darkness within people. He sees people as beings that will eventually enter his world, but sees people with Duel Monster Spirits inside them as "foreign objects" that must be eliminated.

When individuals show fortitude against Nightshroud by using hope and bonds, he will become determined to use anything he has to destroy all hope and bonds that people have within themselves and give himself a better advantage such as using illusions to trick his opponents, using any darkness they have against them, or using his opponent's Duel Partners against them.

According to himself, Nightshroud follows the logic and truth of the world and thus has no actual desires.

In the Japanese version, Darkness uses the word ware (我) to refer to himself and waga (我が) as a possessive pronoun, a very polite and business-like way of doing so. This also speaks to the age of Nightshroud, as this way of speaking is no longer widely used and has not been since Japanese Feudal times.

Nightshroud is a supernatural entity capable of possessing other individuals. He was capable of possessing both Atticus Rhodes and also Yusuke Fujiwara. His means of possession however seem to differ from person to person. In Atticus' case, he was only capable of exerting his control through "Red-Eyes Darkness Dragon" and "Red-Eyes Darkness Metal Dragon" and once Atticus stopped using both cards it ceased any influence Nightshroud had over him. In Fujiwara's case it seemed to be in full control over him, mostly due to his negative feelings. Regardless defeating the possessed individual in a Duel will render the control null.

As seen with the aforementioned "Red-Eyes" monsters it is implied he is capable of upgrading monster cards. Even when possessing individuals Nightshroud is able to do so only through mere parts of him. This enables him to survive despite even losing a Shadow Game in which his soul is gambled.

Being the origin of the World of Darkness he has full control over the dimension, being able to direct and likely create the army of Trueman to assault the human world and imprison nearly all of the planet's inhabitants into his own world. Due to being born out of the darkness of human hearts, Nightshroud is essentially eternal and omnipresent, and therefore cannot be permanently defeated although he can be fended off.

Yusuke Fujiwara learned about Nightshroud before the incident at the Abandoned Dorm, heading to the Dorm's subterranean Duel arena in order to perform a ritual to free it and gain its power. However, Atticus Rhodes, Yusuke's friend, followed him and became caught up in the ritual. Yusuke disappeared, giving Atticus the Nightshroud mask as he no longer needed it. Atticus resisted Nightshroud's influence from then on, until he was forced to embrace it and become its host in order to survive the Dorm's events. He encountered and defeated the Gravekeeper's Chief, obtaining half of a Shadow Charm pendant.

During the Genex Tournament, Atticus uses Nightshroud's Deck in hopes of convincing his friend, Zane, that the dark path taken by a Duelist is a terrible course. As a result of playing his "Red-Eyes Darknesss Dragon", Nightshroud's dark aura begins to resurface. Atticus is able to resist him and nearly win the Duel, but Zane protects himself with his "Power Wall" Trap Card, throwing most of his Deck out in disrespect. This action causes Atticus to believe that Zane can't be helped, and the influence of Nightshroud that lay dormant within the card is once again able to take control of Atticus. Nightshroud immediately destroys Zane's last monster, leaving him with three cards in his Deck, but Zane is still able to bring out his "Cyberdark Dragon", absorb the strength of "Red-Eyes B. Dragon" and destroy "Red-Eyes Darkness Dragon" to win the Duel. As a result, Nightshroud is banished back to his prison following his defeat.

He makes a subsequent return after Jaden accepts Atticus' proposal for a rematch to discover Nightshroud's secrets. Atticus also has nightmares about the day he became Nightshroud's host, which reveals he was not the first or only person to take up Nightshroud's soul. Adding to the mystery, while visiting the arena underneath the Abandoned Dorm, Atticus is attacked by the Duel Monster Spirit Honest, which is disguised as Yusuke Fujiwara, knocking him unconscious in the process. After he recovers, Jaden visits Atticus in the nurse's office and Atticus comes up with the idea of having Jaden Duel Nightshroud (despite the obvious dangers) in order to uncover the entity's origins, which he does. Despite the use of "Red-Eyes Darkness Metal Dragon", Jaden wins again, so Atticus once more stays free from Nightshroud.

Nightshroud later appears to Atticus during the final stages of Trueman's siege at Duel Academy, still in Fujiwara's body, who has completely fallen under Nightshroud's influence and adopted its doctrine. He reveals himself as the mastermind behind the Trueman attacks and is thus the main antagonist of the events. Here, too, his true intentions and motivations are revealed. Nightshroud wanted all humans to be taken into the World of Darkness where they will ultimately give up on their own lives, because while in the dark world, they would be mentally tortured by visions of failing at their hopes and dreams. They would then become a single entity, sharing all joy and all suffering. Nightshroud claimed his actions are justified in that, with no more individuality, humanity's suffering will finally end. He Duels Atticus, who uses Nightshroud's former "Red-Eyes" Deck after modifying it, and shows him a vision during the Duel that leads Atticus to believe he ended it in a DRAW, exploiting the darkness in Atticus's heart. In reality, Nightshroud had used the effect of his "Clear Vice Dragon" to prevent its destruction, allowing it to defeat Atticus and absorb him into the World of Darkness.

Nightshroud is then challenged to a Duel by Jaden and Jesse Anderson. He accepts on the condition that it be a Battle Royal, attempting to search for the darkness in Jesse's heart. Despite the attempts of the "Crystal Beasts" to prevent him from finding out, Nightshroud was able to gleam that Jesse wanted to defeat Jaden, and he appeared to have briefly turned the two against one another, but they instead used their attacks on one another to gain advantages against Nightshroud. Though he defeats Jesse when Jesse sacrifices himself to protect Jaden, Jaden summons "Rainbow Neos" and empowers it with "Honest" to defeat Nightshroud's "Clear Vicious Knight" as well as purge Nightshroud from Fujiwara's body and destroy it.

Nightshroud is defeated.

However, this did nothing to stop Nightshroud, as only part of him was destroyed, and Nightshroud then appeared in its true form. Nightshroud revealed that it created an artificial eclipse over the planet with its own dark world, blocking out the sun and pulling all humans into the dark world. This left Jaden, Pharaoh the cat, and the immaterial Lyman Banner as the last living things on Earth. Nightshroud challenges Jaden to a final Duel, viewing Jaden as a "foreign object" that didn't belong in either world because he held a human spirit and a Duel spirit (referring to Yubel), and thus should be removed. Nightshroud reveals its origins, saying that in every person there is insecurity and inner turmoil: hopes and dreams failing before fear and guilt, creating "darkness", and itself as well. Nightshroud adds a justification to its actions, saying that it's natural for its world to replace the human world. He begins locking Jaden down with his "Darkness," "Zero," and "Infinity" combo. Jaden is able to work his way around it, until Nightshroud Summons "Yubel" from Jaden's Deck. Jaden is able to reclaim Yubel with "Super Polymerization", fusing her with "Elemental Hero Neos" to create "Neos Wiseman," which turns the tables completely in Jaden's favour. Claiming that humanity will lead themselves into ruin, Nightshroud shows Jaden a vision of a ruined future, but Jaden vows not to let it come to pass. Nightshroud is able to take control of the Duel again with "Darkness Seed", allowing him to continually restore his Life Points to 4000. He then calls out his 4000 ATK "Darkness Neosphere" and destroys "Neos Wiseman," but Jaden in turn permanently disables Nightshroud's "Darkness" combo. During the Duel, Jaden reaches out to the humans trapped in the darkness, and reminds them of all of their bonds with each other. The trapped humans eventually free themselves, after which Jaden defeats Nightshroud with "Elemental Hero Divine Neos." Though beaten and forced to return to the World of Darkness, Nightshroud claims that as long as there's darkness in humans, it would surely return, but Jaden counters by claiming that as long as there are bonds between humans, Nightshroud will never return.

When Nightshroud describes the origin of the universe to Jaden, he uses a single card as a metaphor. In Yu-Gi-Oh! ZEXAL, the Numeron Code, a card said to have created all of reality, is a major plot point, possibly what Nightshroud was referring to.